1. Field of the Invention
The present invention relates to a one-part polythiol-type epoxy resin composition having an excellent low-temperature quick curability (i.e., being excellent in quick and low-temperature curability) and a good storage stability or shelf stability (i.e., being good in storage stability).
2. Discussion of the Background
An epoxy resin composition prepared by using a polythiol as a curing agent and a tertiary amine as a curing accelerator has been used as a low-temperature quick-curable epoxy resin composition which can be cured even at such a low temperature of from 0.degree. C. to -20.degree. C., and has been used as an adhesive, a sealer or a casting mold.
However, such an epoxy resin composition has usually quite a short pot life of from several seconds to several minutes after the mixing of the starting ingredients, involving such defect that the mixing of the starting ingredients, and the defoaming and coating of the resulting mixture cannot be conducted for a satisfactorily long period of time. Further, it is poor in workability since a worker has to prepare an epoxy resin composition whenever it is used. Furthermore, the leftover from such a composition cannot be stored, and therefore, has to be discarded, which, in turn, is undesirous from the view point of an energy saving and an environmental problem.
For example, the use of a thiol compound obtained by an esterification reaction of a polyol with a mercapto organic acid as a curing agent for an epoxy resin composition is described in JP-B-41-7236, JP-B-42-26535, JP-B-47-32319, JP-A-46-732 and JP-B-60-21648. However, curing accelerators used in such epoxy resin compositions are liquid amines. Such compositions obtained by using liquid amines as a curing accelerator are very short in their pot life, from several minutes to several tens of minutes, and such very short pot life is quite problematic in the working operation. Under these circumstances, the development of a polythiol-type epoxy resin composition which has a satisfactorily long pot life or which is rendered one-part type to improve a workability has been in high demand.
In order to solve this problem, a method in which an acid anhydride or a mercapto organic acid is added as a curing retarder to prolong a pot life was studied (JP-A-61-159417), but it was not said to be well satisfactory.
JP-A-06-211969 and JP-A-06-211970 describe an example of a polythiol-type epoxy resin composition obtained by using a solid dispersion-type amine adduct latent curing accelerator or a reaction product of a compound having an isocyanate group with a compound having at least one primary or secondary amino group in one molecule as a curing accelerator to improve a pot life. However, the composition is not yet satisfactory in shelf stability, and a low-temperature quick curability as a merit of a thiol-type epoxy resin composition is not brought out sufficiently.